Impact modifiers are widely used to improve the impact strength for thermoplastic compositions with the aim to compensate their inherent brittleness or the embrittlement that occurs at sub zero temperatures, notch sensitivity and crack propagation. So an impact modified polymer is a polymeric material whose impact resistance and toughness have been increased by the incorporation of phase micro domains of a rubbery material. This is usually done due to the introduction of microscopic rubber particles into the polymer matrix that can absorb the energy of an impact or dissipate it. One possibility is to introduce the rubber particles in form of core-shell particles. These core-shell particles that possess very generally a rubber core and a polymeric shell, having the advantage of a proper particle size of the rubber core for effective toughening and the grafted shell in order to have the adhesion and compatibility with the thermoplastic matrix.
The performance of the impact modification is a function of the particles size, especially of the rubber part of the particle, and its quantity. There is an optimal average particle size in order to have the highest impact strength for a given quantity of added impact modifier particles.
These primary impact modifier particles are usually added in form of powder particles to the thermoplastic material. These powder particles are agglomerated primary impact modifier particles. During the blending of the thermoplastic material with the powder particles the primary impact modifier particles are regained and are dispersed more or less homogenously dispersed in the thermoplastic material.
While the particle size of the impact modifier particles in the range of nanometers, the range of the agglomerated powder particles is in the range of micrometers.
Agglomeration during the recovery can be obtained by several processes, as for example, spray drying, coagulation or freeze drying or combination of spray drying and coagulation techniques.
It important to have an impact modifier powder that has no negative influence on the thermoplastic polymer composition. As negative influence, it is understood, for example the color stability, the thermal stability, the hydrolysis stability of the thermoplastic polymer comprising the impact modifier, either on function of the time or the temperature or both.
All these influences might occur due to the architecture of the core-shell but more particularly the impurities and side products employed during the synthesis and treatment of the impact modifier powder. Usually, there is no special purification step of the impact modifier, just a separation of solid versus liquid. Therefore more or less important quantities of any chemical compound (impurities, by-products) employed are still incorporated in the impact modifier. These chemical compounds should not influence the thermoplastic material in a major way as for example degradation of optical and/or mechanical and/or rheological properties with time and/or temperature and/or hygrometry.
An objective of the invention is to have thermoplastic composition containing an impact modifier that has a good compromise between all the properties of the impact modified thermoplastic polymer as having high impact strength, while not reducing the viscosity of the polymer composition and color change at elevated temperatures, due to the influence of impurities or by-products used during the preparation of the impact modifier.
Still another objective of the invention is to have a process for manufacturing an impact modifier, that once the impact modifier is incorporated in thermoplastic composition, the impact modified thermoplastic composition has a good compromise between all the properties of the impact modified thermoplastic polymer as having high impact strength, while not reducing the viscosity of the polymer composition and no important color change at elevated temperatures, due to the influence of impurities or by-products used during the preparation of the impact modifier.
Thermoplastic compositions and impact modified thermoplastic compositions as well as manufacturing processes of core shell impact modifiers and are well known.
WO2009/118114 describes an impact modified polycarbonate composition with a good combination of color, hydrolysis and melt stability. The rubber core is based on polybutadiene. For the preparation of the graft rubber polymer salts of fatty acids, especially of carboxylic acids are used. The yellow index of the compositions given at 206° C. is quite important: 20 or higher.
WO 2009/126637 describes functional MBS impact modifiers synthesized by a multistage emulsion polymerization. At the end the reaction mixture obtained is coagulated in order to separate the polymer. The coagulating treatment is performed by bringing into contact the reaction mixture with a saline solution (calcium chloride or aluminum chloride— CaCl2 or AlCl3) or a solution acidified with concentrated sulfuric acid and then to separate, by filtration, the solid product resulting from the coagulating, the solid product then being washed and dried to give a graft copolymer as a powder.
The document EP 0900 827 describes emulsion grafted polymers, and especially MBS core-shell polymers, which are substantially, free from components that degrade the thermoplastic polymer in that case polycarbonate. Therefore such components are omitted during the synthesis. More generally in order to increase the thermal stability of the impact modified polycarbonate any basic compound has to be avoided during the synthesis of the impact modifier. It is especially insisted on the use of a certain class of surfactants during the emulsion polymerization, especially sulfate- and sulfonate-containing surfactants.
The document US2004/0102564 describes a method for producing thermoplastic molding materials containing rubber. After the polymerization of the rubber polymer a pH buffer system is added to the aqueous phase in order to reduce the mold deposit of the thermoplastic molding. The pH range for the buffer system is large and the choice of the buffer system as well.
All prior art is silent about recovery or isolation process of the powder after the synthesis especially its pH and choice of electrolyte and the performance characteristics of such an impact modifier in a thermoplastic composition.